Awesome! Now the reason for the missing blind plug in the back of the eccentric shaft and the oil drain hole at the lower right of the rear plate finally makes sense! It also makes sense that the engines came stock with a 190°F thermostat, a bypass defeater inside the bypass hole in the waterpump (to allow hot coolant to recirculate at higher impeller speeds), the super hot range spark plugs, the channels in the manifold, the primary channels in the carb spacer, the smallish 90 and 140 fuel jets, the largeish 90 and 160 air bleeds for extra lean high RPM "performance" (weakness), and finally, the very sad primary ports for excellent low end torque, but not much else. That delivery van engine was definitely optimised for low RPM driving. I always suspected as much. Thanks for the pics and info.

 

Given what we've learned so far, I'm somewhat undecided as to what center iron to go with. My choices at the moment are a nice nitrided R5 plate with the longer internal ports and "normal" sized externals, or the previously discussed 12a Y housing.

My concern is this; given I'll be using the intake manifold below (with its 48IDA carb), which has ports that are 43mm tall, would I gain much using the tall 12a ports? Might I be better off with a "matching" R5 type plate? (and I've studied the "Fighting Reversion" thread extensively, that's what got me started on this quest )

 

The nitrided R5 iron in question has the same height intake ports as a "Y" iron...approximately 48mm.
I hope that helps!